Ferrous metals are defined as the conventional grades of steel being denoted by grade according to the American Iron and Steel Institute (AISI) nomenclature which contain carbon and in particular to the steels conventionally designated as plain carbon, alloy, and alloy tool steels. As these grades of steel are raised to elevated temperature for annealing and/or hardening under an ambient furnace atmosphere containing air, hydrogen, water vapor, carbon dioxide and other chemical compounds, it is well known that the surface of the steel will become reactive. Furthermore, in the presence of water vapor, hydrogen, and carbon dioxide in the furnace atmosphere carbon at the surface of the steel will react and be removed from the surface. Removal of carbon from the surface promotes inhomogeneity of the cross-section due to the change in chemistry and crystallography, thus changing the physical properties such as surface hardness and strength of articles which are subsequently fabricated from the ferrous metal. While no atmosphere process can absolutely assure that carbon will neither be added nor deleted from the surface, all atmosphere processes are utilized to minimize either carbon removal (decarburization) or carbon addition (recarburization) to the surface of metal articles undergoing heating for annealing and/or neutral hardening. Excess decarburization or recarburization necessitates making parts to be oversized and then finishing the parts to file dimensions by expensive finishing operations such as machining, grinding, pickling and the like.
With known methods of generating furnace atmospheres for annealing and/or hardening of ferrous metals, it has been found that the atmospheres are generally inconsistent in composition which may lead to decarburization, recarburization, sooting, oxidation, or a combination of these surface phenomena on the metals being treated.
The foregoing problems have ben somewhat alleviated by using a nitrogen based atmosphere to which hydrocarbons, particularly those of a higher order than methane, are used as the carbon control additives. Sooting is a potential problem and the desirable carbon control agent, carbon monoxide, is only produced from air leaking into the furnace or oxide reduction. Sooting is that phenomena which occurs when there is excess carbon potential in the atmospheron which causes carbon deposition on the surface of the articles being treated. Methanol is an improvement over hydrocarbons, but is a liquid at ambient temperature and must normally be metered and injected as such requirig furnace energy for vaporization and dissociation. In some furnaces, such as bell-type annealing furnaces, it is difficult to inject methanol into the furnace and undesirable levels of the decarburizing and oxidizing agents water vapor and carbon dioxide are formed at temperatures below about 1400.degree. F. (760.degree. C.).
U.S. Pat. No. 4,359,351 discloses and claims a process for annealing ferrous metal articles under an atmosphere produced by a methanol and nitrogen mixture injected into a furnace. The specification of U.S. Pat. No. 4,359,351 is incorporated herein by reference.
Dimethyl ether (DME), CH.sub.3 OCH.sub.3, is mentioned in U.S. Pat. No. 4,306,918 as a possible carbon control agent for the carburizing process disclosed by patentees.
U.S. Pat. No. 2,673,821 discloses the use of dimethyl ether as a compound suitable for producing a carburizing atmosphere. Patentee recites that dimethyl ether can be diluted with water to prevent sooting which was a common problem with prior art hydrocarbon atmospheres produced by straight hydrocarbons such as propane, natural gas and the like. However, the water content has to be strictly controlled to prevent loss of carbon efficiency during the carburizing process.
U.S. Pat. No. 2,056,175 contains a lengthy discussion on defining a hydrocarbon compound that will produce an atmosphere which will not have the sooting problems found will methane, ethane, propane, butane, or their derivatives in a carburizing process. This work was continued in regard to carburizing processes and further defined in U.S. Pat. Nos. 2,161,162 and 2,329,896.
U.S. Pat. No. 3,201,290 discloses and claims methods of controlling drip feed carburizing furnaces wherein fluids such as alcohols can be used to produce as carburizing atmosphere.
Lastly, U.S. Pat. No. 1,817,407 discloses process for carburizing using a water vapor-hydrocarbon generated atmosphere.